1. Field of the Invention
The present invention relates to a magnetoresistive element, a magnetic head, and a magnetic recording and reproducing apparatus, and more specifically, to a magnetoresistive element configured to conduct a sense current in a direction perpendicular to a plane of a magnetoresistive film, as well as a magnetic head and a magnetic recording and reproducing apparatus using the magnetoresistive element.
2. Description of the Related Art
In recent years, efforts have been made to reduce the sizes of magnetic recording media while increasing their capacities. Accordingly, the relative speed between a magnetic read head and a magnetic recording medium during read operation has been decreasing. Under the circumstances, expectation to magnetoresistive heads which allow high outputs under the low relative speed has been increasing.
It has been reported that a multilayered film with a sandwich structure including a ferromagnetic layer, a nonmagnetic layer and a ferromagnetic layer successfully produces a high magnetoresistive effect, if the ferromagnetic layers are not anti-ferromagnetically coupled. Specifically, the nonmagnetic layer is referred to as a “spacer layer” or an “intermediate layer”, one of the ferromagnetic layer is referred to as a “pinned layer” or a “magnetization pinned layer”, and the other ferromagnetic layer is referred to as a “free layer” or a “magnetization free layer”. The magnetization of the pinned layer is fixed by applying an exchange biasing magnetic field. The magnetization of the free layer is reversed by an external magnetic field (or signal magnetic field). The magnetization reversal of the free layer changes the relative angle between the magnetization directions of the two ferromagnetic layers sandwiching the nonmagnetic layer between them, making it possible to produce a high magnetoresistive effect. The multilayered film of this type is called a “spin-valve”.
Because the spin-valve can be saturated under a low magnetic field, it is suitable for a read head and has already been put into practical use. However, the magnetoresistive ratio of the spin-valve is limited to about 20%, and a higher magnetoresistive ratio has been required.
The spin-valve includes a CIP (Current-In-Plane) type in which a sense current is supplied in a direction parallel to the film plane and a CPP (Current-Perpendicular-to-Plane) type in which the sense current is supplied in a direction perpendicular to the film plane. The CPP type magnetoresistive element has been reported to exhibit a magnetoresistive ratio about ten times as high as that of the CIP type magnetoresistive element.
In the spin-valve structure, however, the total thickness of the spin-dependent layers is very small and the number of interfaces is also small. Accordingly, if a spin-valve with a structure employed in the conventional CIP type is supplied with a current in the direction perpendicular to the film plane, the element shows a low resistance and thus shows a low output absolute value. When such a spin-valve has a pinned layer and a free layer with a thickness of 5 nm, for example, the output absolute value AΔR for 1 μm2 becomes as small as about 0.5 mΩ μm2. Thus, it is important to increase outputs in order to put a CPP type magnetoresistive element using a spin-valve film to practical use. To achieve this, it is critical to increase the resistance value of a part of the magnetoresistive element which contributes to spin-dependent conduction, and to increase the resistance change. It is also important that the pinned layer and the free layer be made of a material excellent in bulk scattering and interface scattering.
To improve the magnetoresistive (MR) effect, a technique that inserts a resistance increasing layer including an insulator into a spin-valve film has been proposed (see. J. Appl. Phys., 89, p 6943 (2001) or IEEE Trans. Magn., 38, p 2277 (2002)). The spin-valve is composed of a part where the spin-dependent scattering of electrons is caused remarkably (pinned layer/spacer layer/free layer) and a part where the spin-dependent scattering is insignificant (a buffer layer, an antiferromagnetic layer, a protective layer, and the like). Assuming that the resistance of the former part is Rsd and the resistance of the latter part is Rsi, the magnetoresistive ratio (MR ratio) of the spin-valve is expressed by the formula: MR=ΔRsd/(Rsi+Rsd). The insertion of the resistance increasing layer including the insulator into the spin-valve film is intended to obtain improved MR ratio by making Rsd far higher than Rsi.
It has also been reported that the CPP magnetoresistive element is improved in the bulk scattering effect and the interface scattering effect by using Fe50Co50 or a stack of [Fe50Co50/Cu] having a bcc structure as a pinned layer and a free layer (see J. Appl. Phys., 92, p 2646 (2002)).
Further, there has been reported on an attempt to improve the MR ratio by insertion of a spin filter layer of Cu having a thickness of 1 nm (see IEEE Trans. Magn., 38, p 2277 (2002)).